Recyclable CoFe2O4–Ag2O magnetic photocatalyst and its visible light-driven photocatalytic performance

Article abstract of DOI:10.1007/s11164-017-2891-x

A magnetically separable photocatalyst, the composite compound CoFe₂O₄–Ag₂O, was fabricated successfully by a simple precipitation method and used for photodegradation of organic pollutants under visible light (>420?nm) irradiation. Its magnetic separation performance was evaluated. Results showed that the CoFe₂O₄–Ag₂O with 60-wt% content of Ag₂O had the best photocatalytic performance, stability, and magnetic separation performance. Methyl orange, methylene blue, rhodamine B, and phenol can be completely photodegraded by the CoFe₂O₄–Ag₂O photocatalyst in a short period. After five cycles, CoFe₂O₄–Ag₂O kept its performance stability. As prepared, CoFe₂O₄–Ag₂O (60%) has a coercivity of 2500 Oe and a saturation magnetization of 22.45?emu?g^?1; it can be completely separated magnetically in 20?s with magnetic recovery mass rate of 85% under an external magnetic field. This superb photocatalytic performance and separation recovery confirms that the CoFe₂O₄–Ag₂O photocatalyst is a promising candidate for future use in photo-oxidative degradation of organic contaminants.

Full text of DOI:10.1007/s11164-017-2891-x

Res Chem Intermed
DOI 10.1007/s11164-017-2891-x
Recyclable CoFe2O4–Ag2O magnetic photocatalyst
and its visible light-driven photocatalytic performance
Qihui Zeng1 Yingming Zhu1 Wen Tian1
•
•
•
Wei Jiang1 Bin Liang1
•
Received: 8 November 2016 / Accepted: 2 February 2017
Ó Springer Science+Business Media Dordrecht 2017
Abstract A magnetically separable photocatalyst, the composite compound
CoFe2O4–Ag2O, was fabricated successfully by a simple precipitation method and
used for photodegradation of organic pollutants under visible light ([420 nm)
irradiation. Its magnetic separation performance was evaluated. Results showed that
the CoFe2O4–Ag2O with 60-wt% content of Ag2O had the best photocatalytic
performance, stability, and magnetic separation performance. Methyl orange,
methylene blue, rhodamine B, and phenol can be completely photodegraded by the
CoFe2O4–Ag2O photocatalyst in a short period. After five cycles, CoFe2O4–Ag2O
kept its performance stability. As prepared, CoFe2O4–Ag2O (60%) has a coercivity
of 2500 Oe and a saturation magnetization of 22.45 emu g-1; it can be completely
separated magnetically in 20 s with magnetic recovery mass rate of 85% under an
external magnetic field. This superb photocatalytic performance and separation
recovery confirms that the CoFe2O4–Ag2O photocatalyst is a promising candidate
for future use in photo-oxidative degradation of organic contaminants.
Keywords Cobalt ferrite Á Silver oxide Á Visible light Á Photocatalyst Á Magnetic
separation
Introduction
Increasing demand for and shortage of clean water sources have become burning
issues with the progress of humankind. Various methods and techniques have been
proposed to address these issues [1–7]. Among them, photocatalytic oxidation is
&
Wei Jiang
weijiang@scu.edu.cn
1
Multi-Phases Mass Transfer and Reaction Engineering Laboratory, School of Chemical
Engineering, Sichuan University, Chengdu 610065, China
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considered an ideal environmental pollution control technology for color removal
and destruction of persistent organic pollutants due to facile reaction conditions, no
secondary pollution, and direct utilization of sunlight for oxidation reactions
without extra energy consumption [8–11]. TiO2, a typical semiconductor, attracted
attention as a photocatalyst due to its chemical stability, low cost, and nontoxicity
[12]. However, the relatively wide band gap of TiO2 (&3.2 eV) limited its efficient
utilization of solar energy [13]. Developing visible light-driven photocatalysts was
an effective strategy for practically applying photocatalytic oxidation.
Currently, silver compounds like Ag3PO4 [14, 15], Ag2CO3 [16, 18], Ag3AsO4
[19], AgX (X = Cl, Br, I) [20–22], Ag2S [23], and Ag2O [24, 25] have exhibited
superb photocatalytic abilities under visible light irradiation. Typical organic
pollutants such as methyl orange (MO), methylene blue (MB), rhodamine B (RhB),
and phenol can be photodecomposed in a short time [17]. Ag2O can photodegrade
MO completely in 120 s under sunlight irradiation and maintain excellent
photocatalytic activity after six cycles [24]. These facts demonstrate the promising
future of silver compounds, especially Ag2O, for use as highly efficient
photocatalysts.
The expense and potential pollution risk posed by Ag2O use require its recovery
and reuse; however, initial separation of nanoparticulate Ag2O from suspension was
energy-consuming and expensive due to the size of the nanoparticles [26]. Three
energy-efficient strategies were adopted to solve the separation problem of
nanoparticles: immobilization [27], loading on a support [28, 29], and magnetic
separation [30–32]. The first two approaches decreased the difficulty in separation,
but photocatalytic performance decreased due to loss of surface area from
aggregation, loading or coating on supports. Comparably, magnetic separation is
more attractive because the photocatalyst component can be loaded or grown on
small magnetic nanoparticle cores with relatively small loss in surface area. The
photocatalyst forms the active surface layer on the magnetic cores only; hence, the
amount of photocatalyst required is reduced because none is lost to the bulk.
Therefore, the addition of magnetic particles to Ag2O, which improves its
separation efficiency without observable performance loss, is highly attractive.
Cobalt ferrite (CoFe2O4) has been widely used as a magnetic support because of
its high coercivity, moderate saturation, and remarkable chemical stability [33].
CoFe2O4 nanoparticles have been used to fabricate CoFe2O4–graphene composite
photocatalysts, enhancing the photocatalytic efficiency and conducting magnetic
separation in a suspension system [30]. Senapati et al. reported the synthesis of
titania-coated CoFe2O4 by the polymeric precursor method, allowing easy
separation of the photocatalysts after the oxidative process [34]. The hetero-
structure of CoFe2O4 embedded in one-dimensional ZnO photocatalyst showed high
saturation magnetization, which indicated the possibility of recycling the nanos-
tructure [35]. These reports confirmed that CoFe2O4 nanoparticles constitute an
effective magnetic support for obtaining high separation efficiency, adequate
surface area, and excellent photocatalytic performance.
In this study, a magnetically separable CoFe2O4–Ag2O photocatalyst was
designed and fabricated by a simple precipitation method. Morphology and structure
of the CoFe2O4–Ag2O was characterized, and its photocatalytic performance was
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evaluated to determine the best hybrid ratio of the composite by photodegradation
of MO, RhB, MB, and phenol. Its photocatalytic performance and improved
separation efficiency were confirmed by repeating the photodegradation process of
MO and through magnetic separation using an external magnetic field.
Materials and methods
Materials
All analytical-grade reagents, including AgNO3, NaOH, ethanol, Fe(NO3)3Á9H2O,
and Co(NO3)2Á6H2O, were purchased from ChengDu KeLong Chemical Reagent
Co. Ltd. and used directly without further purification.
Synthesis of magnetic CoFe2O4–Ag2O composite photocatalyst
CoFe2O4 nanoparticles were synthesized by the hydrothermal method. Fe(NO3)3-
9H2O (5 mmol) and Co(NO3)2Á6H2O (2.5 mmol) were dissolved in absolute ethanol
(40 mL) to form a clear solution. An appropriate NaOH solution (80 mmol) was
added to the solution until the pH reached 13. After being stirred vigorously for
30 min, the solution was transferred to a 100-mL Teflon-lined stainless autoclave
and maintained at 80 °C for 20 h, and then allowed to cool to room temperature.
The black precipitate was magnetically separated, washed three times with
deionized (DI) water, and dried at 60 °C in air for 12 h to obtain the final
CoFe2O4 product.
A simple precipitation process synthesized CoFe2O4–Ag2O hybrid compounds
with different Ag2O content. To synthesize CoFe2O4–Ag2O with 60 wt% Ag2O
content, 0.28 g CoFe2O4 nanoparticles and 0.6157 g AgNO3 were added into
80 mL of deionized (DI) water with stirring for 2 h. 12 mL of NaOH (0.3 M)
solution was titrated into the suspension at a dropping speed of 3.2 mL min-1 and
the suspension was stirred for 2 h until the initial black color changed to dark
brown. The suspension was then magnetically separated with a permanent magnet.
The separated solid was washed with DI water three times to dissolve any unreacted
raw material or impurities, and dried overnight at 60 °C in air to obtain the final
dark brown powder product.
Analytical methods
Crystal structures of the samples were determined using glancing-angle X-ray
diffraction (XRD; DX-2007 SSC) with a Cu Ka 40 kV/30 mA X-ray source
(k = 0.15406 nm). The X-ray photoelectron spectroscopy (XPS) data (chemical
compositions and valence band spectra) were taken on an XSAM 800 instrument
from Kratos using monochromatic Al Ka radiation and a charge neutralizer. To
compensate for surface charge effects, all binding energies were referenced to the C
1s peak (284.6 eV) arising from adventitious carbon. Scanning electron microscopy
(SEM; JSM-7500F JEOL) and high-resolution transmission electron microscopy
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(HRTEM; FEI Tecnai G2 F20 S-TWIN) were employed to observe the morpholo-
gies and microstructures. Particle size distribution was analyzed by ab JL-6000 type
dry and wet dual-purpose laser particle sizer. UV–visible diffusion reflection
spectrometry (UV–Vis DRS) was performed using a UV–visible spectrophotometer
(Alpha-1506) and fine-grained BaSO4 powder as the standard. Surface areas of
samples were determined by the nitrogen adsorption method with a Brunauer–
Emmett–Teller (BET) analyzer (SSA-3500, Builder, China). The photolumines-
cence (PL) measurements were carried out on a fluorescence spectrometer (F-7000,
Hitachi, Japan) at room temperature. Magnetic characterization was carried out with
a homemade vibrating sample magnetometer. Magnetization curves were measured
at room temperature after applying a maximum magnetic field of 5 T.
Organic dye decomposition
Photocatalytic activity of the samples was evaluated with an OCRS-IV photoreactor
system, which included a 500-W Xe lamp and eight 50-mL quartz test tubes, a cut-
off filter with a specified light wavelength (removes any radiation below 420 nm)
and a water shelter. Photocatalytic evaluation testing was carried out in parallel by
photodegrading organics under visible light illumination without any sacrificial
reagents. Organics used for evaluation included MO (16 mg L-1), RhB
(16 mg L-1), MB (8 mg L-1), or phenol (25 mg L-1) solutions.
In a typical process, 0.1 g of photocatalyst was dispersed in a 10 mL of solution
by stirring in the dark, forming a suspension. Before illumination, the suspension
was kept in the dark for 40 min to reach adsorption equilibrium. In some cases, such
as photodegradation of MB and RhB, the adsorption step in the dark was omitted
because the adsorption rate was significantly slower than the photocatalytic
reaction. The suspension was then exposed under visible light irradiation with an
agitation rate of 250 rpm. After a certain time, an aliquot of the suspension, from
which the photocatalyst had been removed with a magnet, was used to determine its
maximum absorbance peak with a TU-1901 UV–Vis spectrophotometer.
Recyclability of CoFe2O4–Ag2O catalysts was also investigated. After each
round of catalysis, the solid sample was magnetically collected and washed with DI
water. After drying, the sample was used for subsequent photoreaction cycles.
Results and discussion
Catalyst characterization
The XRD diffraction patterns of pure Ag2O, CoFe2O4, and CoFe2O4–Ag2O hybrid
are shown in Fig. 1. For pure Ag2O and CoFe2O4, all the diffraction peaks
determined were assigned by comparison to their corresponding standard materials,
a cubic spinel-type CoFe2O4 (JCPDS 22-1086) and Ag2O (JCPSD 41-1104) with
cubic structure. The characteristic peaks of both materials can be observed in the
spectrum of the CoFe2O4–Ag2O hybrid shown in Fig. 1a. As shown in Fig. 1b, the
diffraction peaks of Ag2O are reinforced and CoFe2O4 are crosscurrent. All the
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Fig. 1 a XRD patterns of the as-prepared CoFe2O4–Ag2O (60%), CoFe2O4, Ag2O. In the CoFe2O4–
Ag2O (60%), peaks marked with (*) correspond to CoFe2O4 and peaks marked with (#) correspond to
Ag2O. b XRD patterns of the as-prepared CoFe2O4–Ag2O with different Ag2O content (range 30–80%)
peaks appear without any significant deviation and no new peaks are observed. This
means that no new substances are generated and no lattice distortions occur due to
the interaction between Ag2O and CoFe2O4; the obtained CoFe2O4–Ag2O hybrid is
a two-solid-phase mixture of pure Ag2O and CoFe2O4.
Morphological structures of CoFe2O4 and CoFe2O4–Ag2O (60%) are further
observed by SEM and TEM (Fig. 2). The SEM image of CoFe2O4 (Fig. 2a) shows
that the as-prepared CoFe2O4 magnetic particles have diameters of 30–50 nm, and
are aggregates of cubic grains the size of *15 nm. After introducing Ag2O by
precipitation, 5–10-nm grains deposited on the surface of the CoFe2O4 aggregation
are observed, forming an encapsulated structure (Fig. 2b). No obvious cubic grain
structures or their junctions are observed. The HRTEM images of the CoFe2O4–
Ag2O (60%) sample (Fig. 2c, d) confirm the existence of Ag2O on magnetic support
CoFe2O4. These show that the outer boundary of the hybrid appears different from
the particle bulk; a number of dark shadows of small grains with diameter of
*5 nm adhere to the edge of giant grey particles. Lattice fringe images of small
grains and bulk determined are 0.25 and 0.27 nm, which are in agreement with the
lattice spaces of (311) of CoFe2O4 and (111) of Ag2O, respectively. The determined
morphology of CoFe2O4–Ag2O (60%) reveals that the Ag2O has tightly wrapped
around the CoFe2O4 nanoparticle aggregation to form larger particles. The BET
result of the CoFe2O4–Ag2O (60%) sample is 24.6812 m3 g-1, which is far larger
than pure Ag2O sample (2.3212 m3 g-1), indicating a potential increase in
photocatalytic performance.
Particle size distribution of CoFe2O4 and CoFe2O4–Ag2O (60%) are determined
and shown in Fig. 3. It can be found that the particle size of the composite sample,
CoFe2O4–Ag2O (60%), ranges from 400 nm to 2000 nm, which is remarkably
larger than of the pure magnetic support, CoFe2O4, between 200 and 400 nm. This
significant increase of particle size should be ascribed to the enwrapping of Ag2O
around the magnetic CoFe2O4 core. This result is in agreement to the SEM results
above.
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Fig. 2 SEM images of a CoFe2O4 and b CoFe2O4–Ag2O (60%). TEM c, d images of CoFe2O4–Ag2O
(60%). The inset shows the lattice fringe images of Ag2O and CoFe2O4, respectively
The band gap energy of CoFe2O4–Ag2O was determined by UV–Vis DRS to
investigate its electronic structure and possible interactions between CoFe2O4 and
Ag2O. The band gap energy was estimated according to Eq. (1):
Eg ¼ 1240=k
ð1Þ
where Eg is the band gap of the semiconductor and k is the threshold wavelength
(the wavelength of the corresponding absorbance edge). The absorbance edge can
be determined by the crossover point of an extended tangent line of the absorption
curve and the x axis. Figure 4 shows that the threshold wavelength of pure Ag2O is
941 nm, with a calculated band gap value of 1.31 eV. The band gap of pure
CoFe2O4 is 1.34 eV, which corresponds well to previous studies [36]. The crossing
point of the CoFe2O4–Ag2O (60%) hybrid is 931 nm, with an estimated band gap of
1.33 eV. This result confirms that there is no noticeable interaction between the
magnetic support and the loaded active species, which is in accordance with the
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Fig. 3 Particle size distribution of a CoFe2O4 and b CoFe2O4–Ag2O (60%)
Fig. 4 UV–Vis diffusion reflection spectrometry of pure Ag2O, CoFe2O4 and as-prepared CoFe2O4–
Ag2O (60%) samples from 200 to 800 nm
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XRD results. Ag2O is only immobilized on the CoFe2O4 surface; therefore, no
significant photocatalytic performance degradation should happen.
Photocatalytic evaluation
MO, a typical persistent organic compound, was selected as the main target to
evaluate the photocatalytic performance of the CoFe2O4–Ag2O hybrids and to
identify the optimal content of Ag2O. Furthermore, other common organic
pollutants like MB, RhB, and phenol were used to assess the performance and
adaptability of the CoFe2O4–Ag2O hybrids.
Adsorption of MO by different CoFe2O4–Ag2O samples in darkness was carried
out to eliminate deviation. About 3% reduction in the initial MO concentration was
observed after 40 min, with no further reduction observed after this. Thus, the time
for adsorption in darkness was set at 40 min. After that, the photodegradation of
MO by CoFe2O4–Ag2O samples with different Ag2O contents under visible light
irradiation was conducted; the results are shown in Fig. 5a.
The results confirm that the pure CoFe2O4 exhibits reaction inertness; MO
concentration remains almost constant throughout the reaction period. However,
with the addition of Ag2O, MO can be completely decomposed within a few
minutes. The photodegradation rate increases proportional to the increase in Ag2O
content. When the Ag2O content of the CoFe2O4–Ag2O hybrid reached 80%, MO
was completely decomposed in 5 min, which is equivalent to the reaction rate of
pure Ag2O. Thus, it was unnecessary to increase Ag2O content further.
The graph in the inset of Fig. 5b shows that the relationship between ln(C/C0)
and irradiation time is linear. This means that the photocatalytic reaction with the
CoFe2O4–Ag2O hybrids follows pseudo-first-order kinetics, which is in agreement
with the common description of dye photodegradation models [8, 37, 38]. The value
of the photoreaction rate constant, k, is equal to the corresponding slope of the
fitting line. The relatively uniform increase of k with increasing Ag2O content is
shown in Fig. 5b. For the CoFe2O4–Ag2O (80%) hybrid, k reached 0.48 min-1
.
Fig. 5 a Effect of different catalysts on photocatalytic degradation of MO: CoFe2O4–Ag2O (mass
content of Ag2O from 30 to 80%). b The rate constant for the photodecomposition of MO on CoFe2O4–
Ag2O photocatalysts with different Ag2O content. The inset reveals the pseudo-first-order phots of ln(C/
C0) versus reaction time with the different Ag2O content
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Optimal Ag2O content of CoFe2O4–Ag2O
In this research, the purpose was to obtain a stable, magnetically separable Ag2O
photocatalyst with superb photocatalytic activity, the lowest viable Ag2O content,
and the highest magnetic recovery. Thus, the recyclability and reusability of the
CoFe2O4–Ag2O hybrids was investigated. The recovery rates and photocatalytic
performances of CoFe2O4–Ag2O (60%), CoFe2O4–Ag2O (70%), and CoFe2O4–
Ag2O (80%) after five cycles were explored and the results are shown in Fig. 6.
As shown in Fig. 6, CoFe2O4–Ag2O (60%) retained over 90% of its initial
photocatalytic activity after 5 runs. MO was completely decomposed in 15 min
during each run. No significant performance decline was observed, which suggests
that CoFe2O4–Ag2O (60%) possesses relatively excellent reusability and photocat-
alytic activities. The reusability of CoFe2O4–Ag2O (70%) was poorer than that of
CoFe2O4–Ag2O (60%); after 5 runs, only 75% of the MO was decomposed in
15 min. CoFe2O4–Ag2O (80%) displayed the worst reusability; 240 min were
required for complete photodegradation of MO after 5 cycles. Despite the first run
showing a very good photodegradation rate, the performance on cycling decreased
too quickly for CoFe2O4–Ag2O (80%) to be considered for further use.
Recyclabilities of the three samples after five runs were determined. The
measured magnetic recovery rates of CoFe2O4–Ag2O (60%), CoFe2O4–Ag2O
(70%), and CoFe2O4–Ag2O (80%) in a magnetic field of 148.7 mT were 85.5, 73.8,
and 43.3%, respectively, with these rates being achieved within 20 s. From this, it
can be inferred that mass loss of photocatalyst could be the main factor of
performance degradation of the hybrids. Considering the three key factors of
magnetic recovery rate, photocatalytic performance stability, and Ag2O usage,
CoFe2O4–Ag2O (60%) is the optimal hybrid for further application.
Fig. 6 Photodegradation of MO in the presence of magnetically recyclable CoFe2O4–Ag2O (60%),
CoFe2O4–Ag2O (70%), and CoFe2O4–Ag2O (80%) photocatalyst under visible light irradiation in cycles
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Fig. 7 Recoveries of CoFeO2O4–Ag2O (60%) with different agitation rates, and the TEM images of
particles in liquid after magnetic separation
The stable photocatalytic performance and high recovery of CoFe2O4–Ag2O
(60%) should be ascribe to the maximum loading amount of Ag2O on the surface of
the CoFe2O4 support. If assuming the surface of the magnetic support has been
completely covered by Ag2O, the redundant Ag2O only exists as the overlapping on
loaded Ag2O with weak adhesion, increasing the effective exposed active surface
slightly. Thus, the photocatalytic performance upgrading of CoFe2O4–Ag2O (70 and
80%) is limited, but their mass loss in recycling is significant.
This assumption can be confirmed by the determined magnetic recovery of
CoFe2O4–Ag2O composite against agitation rate. It can be noted that the magnetic
recovery of CoFe2O4–Ag2O shown in Fig. 7 has a slight decrease with the increase
of the agitation intensity. The residue in the aqueous system after magnetic
separation is nonmagnetic, and be determined as Ag2O since the TEM images show
its octahedron morphology. This result confirms that the detached material of
CoFe2O4–Ag2O composite is pure Ag2O, and results in the performance decay and
mass loss of CoFe2O4–Ag2O.
Characterization of fresh and used CoFe2O4–Ag2O (60%)
To investigate the reason for photocatalytic performance loss of the CoFe2O4–Ag2O
hybrid further, the components, morphology, and magnetic properties of CoFe2O4–
Ag2O (60%) before and after the photocatalysis process were characterized.
Figure 8a shows the XPS spectra of CoFe2O4–Ag2O (60%) before and after five
runs. No significant changes were observed in peak intensity or position, implying
that there was no notable change in hybrid composition or any evidence of
interaction. Detailed valence state analyses of Ag 3d, Co 2p, and Fe 2p were
conducted; Figs. 8b, 6c, d show the narrow scan spectra, respectively. As seen in
Fig. 8b, there are two independent peaks at 368.2 and 374.1 eV, which can be
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Recyclable CoFe2O4–Ag2O magnetic photocatalyst and its…
Fig. 8 a XPS wide spectrum of CoFe2O4–Ag2O (60%). Decomposed XPS b Ag 3d, c Fe 2p, and d Co 2p
assigned to Ag 3d5/2 and Ag 3d3/2 of Ag? , respectively [39]. The Ag valence state
exhibits no change after five cycles, implying the excellent stability of the Ag2O
active component in the CoFe2O4–Ag2O (60%) hybrid.
The main peaks of the Co 2p spectrum (Fig. 8c) are at 796.2 and 785.0 eV and are
assigned to Co 2p1/2 and Co 2p3/2, respectively; the other two peaks at 802.8 and
786.0 eV are attributed to the two orbitals’ shake-up satellite peaks [40]. The Fe
2p spectrum (Fig. 8d) shows peaks at 724.4 and 711.3 eV that are assigned to Fe 2p1/2
and Fe 2p3/2 [41], respectively, and a peak at 718.8 eV that is assigned to the shake-up
satellite of Fe 2p3/2 [42]. The inertness of the magnetic support in CoFe2O4–Ag2O
(60%) during the photocatalytic process was confirmed by the Co 2p and Fe 2p peaks
remaining unchanged. The above XPS results show that the components of CoFe2O4–
Ag2O (60%) remained stable during the photocatalysis process.
The hysteresis loops of CoFe2O4–Ag2O (60%) measured at 300 K before and
after reaction are shown in Fig. 9a. The two hysteresis loops approximately overlap,
showing ferromagnetic behavior at room temperature [43]. A slight decrease of
saturation magnetization of CoFe2O4–Ag2O (60%) was observed from
22.45 emu g-1 initially to 22.24 emu g-1 after 5 runs, which can be ascribed to
testing error. As shown in Fig. 9b, such slight deviation does not significantly
influence the magnetic recovery rate; the recovery rate for CoFe2O4–Ag2O (60%)
was 92.38% after the third magnetic process in a magnetic field of 148.7 mT,
although in the first and second separations, the recovery rates were 96.37 and
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Fig. 9 a Magnetic hysteresis loop of as-prepared CoFe2O4–Ag2O (60%) and after five runs. b Recovery
of five times
94.10%, respectively. The residue after magnetic recovery was determined to be
pure Ag2O. This may be the reason for the performance degradation because the
quantity of Ag2O loaded on the magnetic support decreases on cycling. However,
this relatively high recovery percentage confirmed that the introduction of a
magnetic support could effectively solve the problem of Ag2O nanoparticles, saving
cost and avoiding possible pollution.
The detachment of Ag2O was confirmed by the SEM images before and after five
runs, as shown in Fig. 10a, b. It was observed that some of the protruded structures
on the fresh sample disappear after cycling, which suggests that Ag2O has
dissociated from the support. The BET analysis confirmed that the surface area of
the cycled CoFe2O4–Ag2O (60%) hybrid decreased to 21.3218 m2 g-1, which can
be ascribed to the loss of Ag2O nanoparticles. Thus, it can be ascertained that it is
the loss of free Ag2O content that results in the slight performance degradation of
CoFe2O4–Ag2O (60%).
Electronic structure was also investigated with UV–Vis DRS and PL analysis.
The determined band gap of the cycled CoFe2O4–Ag2O (60%) was the same as
before cycling (Fig. 10c), which verifies its unchanged electronic structure.
However, PL analysis exhibited a marked increase in fluorescence intensity after
five runs (Fig. 10d); this suggests an increase in the recombination rate of photo-
generated electrons and holes, which is in agreement with the slight decrease in
photodegradation rate observed.
Therefore, it is concluded that the combined effect of mass loss and quantum
efficiency decrease in loaded Ag2O caused the performance degradation of
CoFe2O4–Ag2O (60%). However, the stability of CoFe2O4–Ag2O (60%) is still
relatively attractive for actual application.
Extensive application of CoFe2O4–Ag2O (60%)
While CoFe2O4–Ag2O (60%) has shown excellent photocatalytic performance,
stability, and magnetic recovery rate for decomposing MO, it is necessary to
investigate whether it can degrade other organic pollutants. MB, RhB, and phenol
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Fig. 10 SEM images (a, b), UV–Vis (c) and PL (d) of fresh CoFe2O4–Ag2O (60%) and after five cycles
of reaction
Fig. 11 a The degradation rate of MB, RhB, and phenol in darkness and under visible light irradiation.
The inset reveals the Fourier-transform infrared (FT-IR) spectra of CoFe2O4–Ag2O (60%) hybrid (post-
photodegradation). b Photodegradation of MO in the presence of CoFe2O4–Ag2O (60%) under visible
light and near-infrared (NIR) light
were selected to evaluate the performance of CoFe2O4–Ag2O (60%). Results are
shown in Fig. 11.
Figure 11a shows that all of MB and RhB, and *30% of phenol, are adsorbed in
the dark within 12, 8, and 7 h, respectively, which is a much stronger adsorption
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response than observed for the MO. Such strong adsorption could be ascribed to the
large surface area of CoFe2O4–Ag2O (60%), since the adsorption on pure Ag2O
with low surface area is small, with negative f-potential, due to MB and RhB being
cationic dyes [44, 45]. Photodegradation occurred even on exposure to visible light
without an adsorption phase, with MB, RhB, and phenol being completely
decomposed in 30 min, 120 min, and 50 min, respectively. Fourier-transform
infrared spectra (inset in Fig. 11a) confirmed that the adsorbed organics can be
decomposed under light irradiation. This confirms that the strong adsorption plays a
positive role in the photocatalytic process because it can enrich the soluble organics
on the surface of the photocatalyst. Overall, CoFe2O4–Ag2O (60%) shows excellent
photocatalytic activity for degrading MB, RhB, and phenol, thereby confirming its
wide applicability.
MO photodegradation was also carried out under near-infrared (NIR) light
irradiation to verify the photocatalytic ability over a wider spectrum including
longer wavelengths. Figure 11b shows that the MO was completely decomposed in
120 min under NIR illumination. Although the photodegradation rate is slower, this
shows that CoFe2O4–Ag2O (60%) works over a wider spectrum than traditional
photocatalysts, such as TiO2. This attractive property can be ascribed to the narrow
band gap of Ag2O.
Conclusion
In this research, a range of magnetically separable CoFe2O4–Ag2O hybrid
photocatalysts of different Ag2O contents were prepared and evaluated. CoFe2O4–
Ag2O hybrids exhibited excellent photocatalytic performance under visible light
and NIR light irradiation. Typical persistent organics, such as MO, MB, RhB, and
phenol, were completely photodecomposed with the optimized hybrid CoFe2O4–
Ag2O (60%) within 120 min. This hybrid photocatalyst can be effectively separated
magnetically without considerable mass loss, and can be reused without significant
performance degradation. This excellent photocatalytic performance and separation
recovery confirms that the CoFe2O4–Ag2O photocatalyst is a promising compound
for future application for photo-oxidative degradation of organic contaminants.
Acknowledgements We are thankful for the financial support provided by the National Natural Science
Foundation of China Project (no. 21476146).
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